Mendozite

Mendozite is a sulfate mineral, one of the alum series, with formula NaAl(SO₄)₂·11H₂O. It is a hydrated form of sodium aluminium sulfate (soda alum).

Mendozite
General
Category	Sulfate minerals, alum series
Formula	NaAl(SO4)2·11H2O
IMA symbol	Mz[1]
Strunz classification	7.CC.15
Dana classification	29.5.4.1
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Identification
Formula mass	440.26 g/mol
Color	colorless
Crystal habit	prismatic, pseudo-rhombohedral
Cleavage	{100} good {001} indistinct {010} indistinct
Mohs scale hardness	3
Luster	vitreous
Streak	white
Diaphaneity	transparent to translucent
Density	1.74 g/cm3
Optical properties	biaxial (-)
Refractive index	nα = 1.449 nβ = 1.461 nγ = 1.463
Birefringence	δ = 0.014
2V angle	56° (measured)
Solubility	soluble in water
Alters to	tamarugite
References	[2][3][4][5]

It was discovered in western Argentina in 1868, probably near San Juan. The exact location has been lost, but was described as "San Juan, near Mendoza", and it is the latter city that give the mineral its name.^[3] It occurs in evaporites, presumably from the oxidation of sulfide minerals in the presence of clays.^[3] It is very soluble in water, and so can only be found in dry regions: however, in can still effloresce (lose water of crystallisation) in extremely arid climates, altering to tamarugite (the hexahydrate).^[4]

Bibliography

• Palache, P.; Berman H.; Frondel, C. (1960). "Dana's System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. (Seventh Edition)" John Wiley and Sons, Inc., New York, pp. 469-471.